In general, magnetostrictive materials may be formulated to also contain magnetic material, such as a cobalt or nickel ferro-magnetic alloy, thereby biasing the magnetostrictive material, altering its shape response in the presence of an additional external magnetic field. This biasing technique is blended into the magnetostrictive material, permanently fixing its shape response. Whether biased with a magnetic material or not, the magnetostrictive material is still called a magnetostrictive material.
Magnetostrictive materials, particles in particular, may be used inside a porous medium to change the porosity or permeability properties of the medium. Such materials or particles may be called magnetopropants, alluding to the effect of changing the interstitial volume by exerting force on the matrix of the medium by the change in the shape of the magnetostrictive material. To effect shape change, the magnetostrictive material must be exposed to a magnetic field. It is not always practical to supply a controllable magnetic field of sufficient strength at locations where it is desired to actuate the magnetostrictive material. For instance, it is difficult to supply a controllable magnetic field in a geologic formation at a distance far from a well bore.